Altering Na-ion solvation to regulate dendrite growth for a reversible and stable room-temperature sodium–sulfur battery

Abstract

Unwarranted reactivity of sodium with electrolytes leads to their constant consumption and dendrite growth, causing sodium-metal batteries to fail prematurely. The interface and electrolytes are often engineered to boost stability and reversibility; however, designing and understanding the correlation between the interphase and electrolyte dynamics remained challenging. Here, we report an alloying-type electrolyte additive, i.e., bismuth triiodide (BiI₃), to alter Na⁺-ion solvation and its redox dynamics on the sodium metal surface. Theoretical calculation assisted experimental characterization reveals that the additive changes the local solvation shell dynamics and improves the Na ion kinetics by reducing binding energy. In addition, the formation of an alloy interphase over a metal anode realizes a dendrite-free sodium-metal anode, which even retains its interfacial integrity after long-term cycling in both Na-symmetric cells (for over 1600 hours at 1 mA cm⁻²) and Na-metal batteries with sulfurized polyacrylonitrile (SPAN) as a cathode (for over 250 cycles). Electrolyte engineering through alloying additives alters both solvation dynamics and interfacial properties, pointing us toward a new direction to harness sodium metal as the most promising anode.